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Choi E.,Hongik University | Kim D.,Hongik University | Lee J.-H.,Daegu University | Ryu G.-S.,Structural Engineering Research Institute
Composites Part B: Engineering | Year: 2017

The aim of this study is to assess the pullout resistance of superelastic shape memory alloy (SMA) short fibers with different end shapes, which provide different anchoring actions, through monotonic and hysteretic pullout tests. For this study, NiTi superelastic SMA wire with a diameter of 1.0 mm was prepared and cut by a length of 40 mm to make SMA short fibers. Four types of SMA fibers with different end shapes were manufactured namely: 1) straight end shape, 2) crimped end, 3) bended end with L-shape, and 4) spearhead end. The straight end-shaped fiber was one without any anchoring action on the end part. The crimped fiber had grooves on the two sides manufactured by crimping an end part of 5 mm. The end-bended fiber had an L-shaped end with a 30° bending angle. The fiber with a spearhead was manufactured by pressing the end part of 5 mm. The pullout tests of the fibers from the mortar matrix were first monotonically conducted with displacement control, and the hysteretic pullout behavior was obtained by 4 cyclic loadings. The L-shaped fibers increased the pullout resistance significantly compared with the straight and crimped fibers. However, they did not reach the upper plateau stress of the SMA fiber to induce state transformation. Only the fibers with a spearhead end exceeded the stress for the state transformation because they provided sufficient anchoring resistance due to the spearhead. The maximum pullout resistance of the spearhead fiber was 3.74 times that of the L-shaped fiber. Moreover, they showed flag-shaped behavior during the hysteretic tests. © 2016 Elsevier Ltd

Park J.-J.,Structural Engineering Research Institute | Yoo D.-Y.,Hanyang University | Park G.-J.,Structural Engineering Research Institute | Kim S.-W.,Structural Engineering Research Institute
Materials | Year: 2017

In this study, the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) is examined as a function of fiber length and volume fraction. Straight steel fiber with three different lengths (lf) of 13, 19.5, and 30 mm and four different volume fractions (vf) of 0.5%, 1.0%, 1.5%, and 2.0% are considered. Test results show that post-cracking flexural properties of UHPFRC, such as flexural strength, deflection capacity, toughness, and cracking behavior, improve with increasing fiber length and volume fraction, while first-cracking properties are not significantly influenced by fiber length and volume fraction. A 0.5 vol % reduction of steel fiber content relative to commercial UHPFRC can be achieved without deterioration of flexural performance by replacing short fibers (lf of 13 mm) with longer fibers (lf of 19.5 mm and 30 mm). © 2017 by the authors.

Kim H.-J.,Korea Agency for Infrastructure Technology Advancement | Ham J.,Inha University | Park K.-T.,Structural Engineering Research Institute | Hwang W.-S.,Inha University
Steel and Composite Structures | Year: 2017

This study intends to improve the structural details of the anchors in the conventional CFT column-to-foundation connection. To that goal, finite element analysis is conducted with various design variables (number and embedded length of deformed bars, number, aspect ratio, height ratio and thickness ratio of ribs) selected based upon the results of loading test and strength evaluation. The finite element analysis is performed using ABAQUS and the analytical results are validated by comparison with the load-displacement curves obtained through loading test applying axial and transverse loads. The behavioral characteristics of the numerical model according to the selected design variables are verified and the corresponding results are evaluated. Copyright © 2017 Techno-Press, Ltd.

Min J.,Structural Engineering Research Institute | Min J.,Korea University | Lee B.,Structural Engineering Research Institute | Lee J.-S.,Structural Engineering Research Institute
Construction and Building Materials | Year: 2016

Concrete that is used in neutron generating facilities, such as fission reactors, becomes radioactive owing to neutron irradiation. Low-activation and neutron shields are critical concerns at the dismantling stage with requirements for radioactive waste management. To enhance these parameters, synthetic resins-based mortar was investigated in this study. The mass of hydrogen is almost the same as that of a neutron, and it effectively retains neutrons by increasing the probability of elastic scattering. It is well known that synthetic resins have numerous hydrogen atoms. Resins containing mortar mixtures were investigated to understand the effect of resin on the mechanical properties of mortar as well as on its neutron-shielding performance. The experimental results showed that the neutron dose equivalent to a resin-based mixture decreased by 63.86% as compared to that of conventional mortar, although the mechanical properties need to be enhanced using suitable special treatments. It indicated significant potential to reduce not only the thickness of the neutron shields but also the amount of radioactive waste. © 2016 Elsevier Ltd

Jeong Y.-J.,Structural Engineering Research Institute | Park M.-S.,Structural Engineering Research Institute | You Y.-J.,Structural Engineering Research Institute
Structural Engineering and Mechanics | Year: 2016

In this study, wave force tests were carried out for the four types of offshore support structures with scale factor 1:25 and wave forces to the support structure shapes were investigated. As the results of this study, it was found that, as the wave period increased at the normal wave condition, wave force decreased for the most cases. Extreme wave force was affected by the impact wave force. Impact wave force of this study significantly effect on Monopile and slightly on GBS and Hybrid type. Accordingly, Hybrid type indicated even lower wave force at the extreme and irregular wave conditions than the Monopile although Hybrid type indicated higher wave force at the normal wave condition of the regular wave because of the larger wave area of wave body. In respects of the structural design, since critical loading is extreme wave force, it should be contributed to improve structural safety of offshore support structure. However, since the impact wave force has nonlinearity and complication dependent on the support structure shape, wave height, wave period, and etc., more research is needed to access the impact wave force for other support structure shapes and wave conditions. Copyright © 2016 Techno-Press, Ltd.

Heo S.,Inha University | Koo W.,Inha University | Park M.-S.,Structural Engineering Research Institute
International Journal of Structural Stability and Dynamics | Year: 2016

A fast, reliable and optimized numerical procedure of the hydrodynamic response analysis of a slender-body structure is presented. With this method, the dynamic response and reliability of a six-leg jack-up-type wind turbine installation vessel under various environmental conditions is analyzed. The modified Morison equation is used to calculate the wave and wind-driven current excitation forces on the slender-body members. The Det Norske Veritas (DNV) rule-based formula is used to calculate the wind loads acting on the superstructure of the jack-up leg. From the modal analysis, the natural period and standardized displacement of the structure are determined. The Newmark-beta time-integration method is used to solve the equation of motion generating the time-varying dynamic responses of the structure. A parametric study is carried out for various current velocities and wind speeds. In addition, a reliability analysis is conducted to predict the effects of uncertainty of the wave period and wave height on the safety of structural design, using the reliability index to indicate the reliability of the dynamic response on the critical structural members. © 2016 World Scientific Publishing Company

Lee H.-S.,Hanyang University | Jang H.-O.,Hanyang University | Cho K.-H.,Structural Engineering Research Institute
Materials | Year: 2016

This study set out to derive the optimal conditions for ensuring the monolithicity of ultra-high-performance concrete (UHPC). Direct shear tests were performed to examine the influence on the bonding shear performance. The experimental variables included tamping and delay, which were set to 0, 15, 30, and 60 min. SEM and XRD analyses of the microstructure and composition were performed. The direct shear tests showed that the bonding shear strength was enhanced by the addition of tamping. For the normal-strength concrete (NSC), it is thought that a monolithicity of around 95% can be attained with a cold joint formation delay up to 60 min. In contrast, while the normalized bonding shear strength reduction of UHPC with a delay of 15 min was the lowest at around 8%, a dramatic degradation in the bonding shear performance was observed after 15 min. XRD analyses of the middle and surface sections revealed the composition of the thin film formed at the surface of the UHPC and, as a result, the main component appeared to be SiO2, which is believed to be a result of the rising of the SiO2-based filler, used as an admixture in this study, towards the surface, due to its low specific gravity. © 2016 by the authors.

Kim T.-H.,Structural Engineering Research Institute | Park K.-T.,Structural Engineering Research Institute
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2016

The recent constructed structures are featured by the combination of their functions and shapes as well as by their enlarged dimensions, which increase the demand for Structural Health Monitoring technology. Since every structure bears unique dynamic characteristics and is exposed to diverse external forces, various methods are studied to monitor the health of the structure. This study applies the Hilbert-Huang transform, the variance analysis and the edge detection method on the acceleration response of the structure to identify the initiation time of the abnormal behavior in which the structure experiences abnormal vibration. A scaled cable-supported bridge model is fabricated and subjected to cable failure test from which data before and after the occurrence of the abnormal behavior are acquired and compared to validate the proposed anomaly-extraction technique. © 2016 SPIE.

You Y.-J.,Structural Engineering Research Institute | Jeong Y.-J.,Structural Engineering Research Institute | Park M.-S.,Structural Engineering Research Institute
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2016

A recent trend is installing wind power systems out in the ocean. Many countries either have already constructed or are planning to construct huge wind farms competitively. Up until now, substructures for offshore wind power systems have mostly been built on a mono-pile type due to relative simplicity for construction and cost-effectiveness. However, the mono-pile type substructures may be faced with limitations of size or construction in the near future as the wind power turbines get bigger and the installation sites need to be farther from the shore in order to obtain high-quality wind power. A hybrid type substructure was designed to overcome these limitations. This substructure consists of multi-piles and a concrete base. The concrete base acts as a weight in a gravity type, which has good overturning stability, and the multi-piles plays a role of reducing the wave force, thereby reducing overturning moment. This study is about an experimental evaluation for overturning stability of the hybrid type substructure. A total of three specimens with a size scale of 1/25 including mono-pile, gravity, and hybrid types were fabricated and tested in a basin with 2 m, 3 m, and 100 m for width, depth, and length, respectively. A total of six regular waves were applied to these specimens. The test results of hybrid type were compared with those of mono and gravity types. The hybrid type substructure received less force by the waves than mono and gravity types, and this means that the hybrid type substructure secured better overturning stability than other types. © Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE).

Park M.-S.,Structural Engineering Research Institute | Jeong Y.-J.,Structural Engineering Research Institute | You Y.-J.,Structural Engineering Research Institute
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2016

The offshore wind energy has gained attention from many countries to find alternative and reliable energy sources. Therefore, many offshore wind farms are in the planning phase. In order to increase the gross generation of wind turbines, the size of a tower and a rotor-nacelle becomes larger. In other words, the substructure for offshore wind turbines is strongly influenced by the effect of wave forces as the size of substructure increases. In the present study the hybrid substructure, which is composed of a multi-cylinder having different radius near free surface and a gravity substructure at the bottom of multi-cylinder, is suggested to reduce the wave forces. In addition, since a large offshore wind turbine has a heavy dead load, the reaction forces on the substructure become severe, thus very firm foundations should be required. Therefore, the dynamic pile-soil interaction has to be fully considered. In the present study the ENSOFT Group V7.0 is used to calculate the stiffness matrices on the pile-soil interaction conditions. Using the stiffness matrices and the loads at TP, which is obtained from GH-Bladed, the structural analysis of the hybrid substructure is carried out through ANSYS ASAS. The structural strength and deformation is evaluated to investigate an ultimate structural safety and serviceability of hybrid substructure for various soil conditions. The first few natural frequencies of substructure are heavily influenced by the wind turbine. Therefore, the modal analysis is carried out through GH-Bladed to investigate the resonance between the wind turbine and the hybrid substructure. © Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE).

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